Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. Some LEDs have higher light conversion efficiencies and longer lifetimes than the conventional sources.
Unfortunately, LEDs have a number of problems that have inhibited their widespread use as replacement light sources. First, the power output of a single LED is significantly lower than that of a conventional light source, and hence, a number of LED dies must be utilized to provide a source that can deliver light at intensities consistent with the light levels provided by conventional light sources. In addition, some LED light sources utilize LEDs that emit different colors, e.g. red, green, and blue, to produce light that is perceived by a human observer to be of a specific color that is different from that of the individual LEDs. Hence, inexpensive packaging that can hold a number of dies is required.
Second, the high light conversion efficiencies that make LEDs attractive as replacement candidates depend on providing an environment in which the heat generated by the LED is efficiently removed so that the LED is not subjected to high temperatures. For the purposes of this discussion, the light conversion efficiency of a light source is defined to be the amount of light generated per watt of electricity consumed by the light source. The light conversion efficiency of the LEDs that are currently available decreases rapidly with increasing temperature. In addition to reducing the light conversion efficiency, heat also shortens the lifetime of the LED and can lead to premature total device failure. Hence, any packaging arrangement must provide an efficient path for removing heat from the dies.
Lead frame packages are attractive from a cost point of view. However, lead frame packages that accommodate multiple LEDs and provide sufficient heat dissipation are not available for high power dies. These packages typically rely on moving heat from the LED to an outside heat dissipating surface by placing the LEDs on the internal portion of the leads and relying on the lead to move the heat to the heat dissipating surface. Unfortunately, the heat paths tend to have too high of a thermal resistance, and hence, the heat dissipation is not sufficient for many applications.